The aims of the research in the past year were to investigate if the changes in the receptor expression on tumor induced angiogenic blood vessels could be used as a biological marker during a course of therapy to develop a rational-based dosing schedule for an efficient combined therapy. We previously found that an integrin alpha(v)beta(3) receptor antagonist, In-111-DOTA-E-c(RGDfK)2 rapidly localized in tumor via the receptor-mediated binding on neovasculature and was retained in the tumor for a prolonged time whereas it rapidly cleared from the circulation and the whole-body through renal excretion. We also found that compared to control mice without drug treatment, Taxol treatment initially decreased the level of the receptor on tumor neovasculature but drastically increased the receptor level prior to the rebounding of tumor-growth. The treatment with a tumor-specific immunotoxin SS1P increased the receptor level at both the initial and late time points. In the past year, we continued these studies and addressed two questions 1) if we could moderate the enhanced receptor expression by an Avastin or additional Taxol treatment and 2) if the changes in the receptor expression on tumor induced angiogenic blood vessels could be used as a biological marker to develop a rational-based dosing schedule for an efficient combined therapy. Methods: Groups of nude mice (n = 5-10 per time point), implanted s.c. with A431/K5 tumor cells expressing mesothelin and not alpha(v)beta(3), were treated with taxol alone i.p at 50 mg/kg on day 0, mesothelin-specific immunotoxin SS1P alone i.v. at 0.2 mg/kg on days 0 and 2, an anti-VEGF antibody Avastin alone i.v. at 5 mg/kg or the two agents together. The tumor size was 150 mm3 on day 0 and was measured daily for one week and thereafter 3 times per week. For the assessment of therapeutic response, the mice received i.v. In-111-DOTA-E-c(RGDfK)2 (2.0 micro-Ci/<0.1 micro-g) in 0.2 ml of PBS containing 1% BSA on days 1 and 4 for the taxol alone groups and on days 1 and 3 for the immunotoxin alone groups, and were euthanized at 1 or 2 hr post-injection for biodistribution. For the combination therapy groups involving Taxol treatment on day 0 and SS1P treatment on days 1 and 3, the mice were injected i.v. with the radioligand on days 2 and 4, and were euthanized at 1 or 2 hr post-injection of the radiolabel. For the groups treated with Taxol on day 1 and Avastin on day 2 or Taxol on day 0 and an additional Taxol on day 3, the receptor levels were measured on day 4. Results: The treatment with Taxol and SS1P at a dose which arrests the tumor growth always increased the receptor expression prior to the rebounding of tumor growth. Interestingly, the additional Taxol treatment on day 3 moderated the receptor expression on day 4 to the level of the control, indicating that Taxol indeed decreases the receptor expression at one day postinjection. This moderation of the receptor expression by the additional Taxol treatment was comparable to that of a combined Taxol and Avastin treatment, suggesting that the increase in v3 expression prior to tumor re-growth stemmed from the re-induction of angiogenesis by over-expression of vascular endothelial growth factor (VEGF). Conclusion: The changes in the v3 expression level on neovasultaure could provide a useful time window important in developing a rational-based dosing schedule for an effective combined therapy. A combined therapy involving Taxol treatment followed by SS1P treatment within a time window of reduced v3 expression produced a synergistic effect in the shrinkage of tumors. The increased v3 expression prior to tumor re-growth suggests that an addition of anti-angiogenic therapy would be beneficial to the combined therapy involving Taxol and SS1P. The results also suggest that a combined therapy involving Taxol and SS1P in alternating sequence may provide a beneficial effect in prolongation of a median survival time.